1. Field
The invention relates to a nuclear fuel rod including a boost zone and a fuel bundle assembly including the nuclear fuel rod.
2. Description of the Related Art
Referring to FIG. 1, a conventional operating environment for fuel rods and fuel assemblies is illustrated in the sectional view, with parts cut away, of a boiling water nuclear reactor pressure vessel (RPV) 10 for clarity. The reactor pressure vessel 10 has a generally cylindrical shape and is closed at one end by a bottom head 12 and at its other end by a removable top head 14. A side wall 16 extends from the bottom head 12 to the top head 14. The side wall 16 includes a top flange 18 on which the top head 14 is attached. A cylindrically shaped core shroud 20 surrounds a reactor core 22. The shroud 20 is supported at one end by a shroud support 24 and includes an opposed removable shroud head 26. Heat is generated within the core 22, which includes fuel bundles 40. The fuel bundles 40 include a plurality of fuel rods having fissionable material. For example, a conventional fuel bundle 40 may include a 10×10 matrix of fuel rods.
A conventional fuel rod 100, as illustrated in FIG. 2, may have a length of about 150 inches but may have lengths of about 120 inches, 133 inches, 138 inches, 139 inches, and 145 inches. The conventional fuel rod 100 is filled with sintered pellets of nuclear fuel, for example, natural and/or enriched uranium. The nuclear fuel may be arranged within the conventional fuel rod so that different regions (zones) within the fuel rod 100 include sintered pellets having different enrichments.
The conventional nuclear fuel rod illustrated in FIG. 2 includes three different zones 110, 120, and 130. The first and third zones 110 and 130 span approximately the first and last six inches of the fuel rod and include natural uranium having 0.71% U235 by weight whereas the second zone 120 includes enriched uranium having, for example, 2.4% U235 by weight. Although the conventional fuel rod illustrated in FIG. 2 includes a second zone 120 having an enrichment of 2.4%, those skilled in the art would readily understand that the second zone 120 of the conventional fuel rod 100 may include uranium enriched up to 5% U235 by weight.
It is well known that reactor cores are loaded with excess fuel and thus excess reactivity in order to maintain power through power cycles extending several months. In order to compensate for this, burnable poisons are incorporated in the fuel bundles along with the fuel rods. A burnable poison is a neutron absorber, which is converted by neutron absorption into a material of lesser neutron absorbing capability. A well known burnable poison is gadolinium, normally in the form of gadolinia. It is also known that power distribution within the reactor core is skewed toward the lower regions of the core. In order to compensate for this, the burnable poisons are incorporated in the fuel bundles along with the fuel rods in the lower regions of the core.
FIG. 3 illustrates a conventional rod 200 containing gadolinia. Like the conventional fuel rod 100 illustrated in FIG. 2, the rod 200 also includes three zones 210, 220, and 230. Like the conventional rod 100, the first and third zones 210 and 230 of the rod 200 include natural uranium. However, the second zone 220 may be either partially or completely filled with enriched uranium containing gadolinia. In the event the second zone 220 is partially filled with gadolinia the remainder of the second zone 220 is filled with enriched uranium.
FIG. 4 is an example of a conventional fuel bundle 40. As shown in FIG. 4, the fuel bundle 40 encloses a plurality of fuel rods 100. The fuel rods 100 within the fuel bundle 40 are supported at a lower end by a lower tie plate 42, in the middle by a interim spacer 44, and at the top by an upper tie plate 46. The fuel bundle 40 also includes a fuel channel 50, which encloses the plurality of fuel rods 100, and a bail handle 52 for transporting the fuel bundle 40. At the bottom of the fuel bundle 40 is a nosepiece 54 which allows a coolant to flow into and through the fuel bundle 40. In addition to fuel rods 100, the conventional fuel bundle 40 typically includes water rods near the center of the fuel bundle 40 that allow the coolant to flow through for neutron moderation. The conventional fuel bundle 40 also includes rods 200 containing gadolinia.
FIG. 5A illustrates a cross-section of a fuel bundle 40 which includes an array of conventional fuel rods (F1, F2, F3, F4 and P1), an array of fuel rods containing gadolinia (G1, G2, and G3), and two water rods. The profiles of each of the fuel rods and the water rods in the fuel bundle 40 as illustrated in FIG. 5A are provided in FIG. 5B. For example, the F1 fuel rod, as illustrated in the FIG. 5A at grid point A-1 (and illustrated in FIG. 5B), includes a natural uranium blanket corresponding to the first zone 110 of the conventional fuel rod 100 illustrated in FIG. 2. In FIGS. 5A and 5B, the aforementioned blanket occupies the bottom six inches of the fuel rod F1 (i.e. bottom six inches includes natural uranium having 0.71% U235 by weight). The middle portion of fuel rod F1 (corresponding to the second zone 120 of the conventional fuel rod 100 illustrated in FIG. 2) is about 138 inches long and includes enriched uranium (i.e. uranium including 2.4% U235 by weight). The top six inches of fuel rod F1 (corresponding to the third zone 130 of the conventional fuel rod 100 illustrated in FIG. 2) includes natural uranium having 0.71% U235 by weight. As discussed above, the bottom six inches of each of the fuel rods in the conventional bundle are filled with natural uranium having 0.71% by weight U235.
FIGS. 5A and 5B also illustrate an example of a conventional fuel rod that includes gadolinia. For example, G1 located at grid point D-4 (and having a profile illustrated in FIG. 5B) illustrates a fuel rod having a lower blanket of natural uranium containing, by weight, 0.71% U235. The lower blanket corresponds to the first zone 210 illustrated in FIG. 3. The middle portion of G1 includes fuel containing 4.9% enriched uranium. The middle portion corresponds to the second zone 220 illustrated in FIG. 3. As illustrated, the middle portion of G1 includes a portion which includes 8% gadolinia by weight and a portion that does not include any gadolinia. The top six inches of G1 is filled with natural uranium having 0.71% U235 by weight. This top blanket of natural uranium corresponds to the third zone 230 illustrated in FIG. 3.